The present invention relates to gas phase polymerization of ethylene using a supported chromium-titanium catalyst, and especially to production of polyethylene resins suitable as feedstock for making a blow molding resin.
Polymerization of ethylene to solid polymers dates back over 50 years. However, early ethylene polymerization was carried out at high pressures, in excess of 1,000 psig, and the costs of production were relatively high. In the early 1950's, Karl Ziegler and others disclosed catalysts and processes that produced polyethylene at much lower pressures. Since that time, the production of polyethylene has grown immensely, and there has been a succession of improvements and alternate catalysts and processes disclosed.
Ziegler and his co-workers disclosed that polyethylene could be made at low pressure using binary mixtures of metal alkyls and transition metal salts, such as aluminum triethyl and titanium tetrachloride.
Hogan and Banks, at Phillips Petroleum, disclosed that polyethylene and other polyolefins could be made at moderate pressures using catalyst comprising chromium compounds on a silica support, see U.S. Pat. No. 2,825,721. Polyethylene is produced in the Phillips' process in a slurry system (sometimes called "particle form process") . In the slurry system, a slurry of isobutane or isopentane with suspended catalyst and polymer particles is circulated in a reactor loop. As larger polymer particles are formed, they enter a settling zone and are withdrawn from the reactor loop.
In contrast to the slurry system, various gas phase systems have been disclosed. For example, in the Union Carbide process, a fluidized bed is maintained by circulated gaseous ethylene. The term "gas-phase polymerization" has been called a misnomer because the polymerization is believed to occur on or within solid particles of polyethylene contained in the reaction zone. But this gas phase terminology is useful because it contrasts to the slurry system and it is descriptive of the mostly gas phase conditions in the reaction zone.
Naphtachimie has also disclosed gas phase ethylene polymerization, see, for example, U.S. Pat. No. 3,922,322.
Still further, Amoco has described a horizontal mechanically stirred gas phase system, see, for example, U.S. Pat. No. 3,965,083; U.S. Pat. No. 3,971,768; and U.S. Pat. No. 4,129,701.
Catalysts used in the gas phase processes and in slurry systems include hexavalent chromium oxide deposited on a silica carrier as described in the previously mentioned U.S. Pat. No. 2,825,721; chromium oxide-titanium oxide-silica as described in U.S. Pat. No. 3,622,521; silyl chromate on silica as described in U.S. Pat. No. 3,324,101; bis-(cyclopentadienyl)chromium (II) on silica as described in U.S. Pat. No. 3,709,853; organo-aluminum-titanium halide catalyst of the Ziegler type; and chromium oxide-titanium oxide-fluoride on a silica support as described in U.S. Pat. No. 4,011,382.
U.S. Pat. Nos. 3,780,011 and 4,041,224 also disclose a catalyst containing chromium and titanium compounds on a support such as silica. The '224 patent theorizes that after the catalyst is activated in the presence of oxygen, the catalyst has chromium atoms bridged to titanium atoms by oxygen in the form of Cr--O--Ti.
The amount of chromium and titanium used in the above-mentioned Cr--Ti--Silica catalysts frequently is stated to be in a rather broad range of about 1/10 weight percent to 10 weight percent. For example, U.S. Pat. No. 4,041,224 states the amount of chromium is preferably between 0.1 and 10 weight percent and that "a catalyst composition containing 1 weight percent chromium is highly active; consequently, there is ordinarily no reason to use a larger percent." The quantity of titanium is stated in the '224 patent to preferably be 0.5 to 8 weight percent based on the weight percent of the support material, and more preferably in the range 2 to 6 weight percent.
U.S. Pat. No. 4,540,755, entitled "Inclusion of Oxygen in the Polymerization of Olefins", discloses the polymerization of olefins with an oxygen containing gas using a supported organophosphoryl-chromium oxide catalyst. The reaction is carried out in the '755 patent in the liquid phase, such as in a slurry system. The inclusion of oxygen is said to result in improved physical properties for the resins produced, such as polyethylene resins. The ratio of HLMI to MI trends generally downward with increasing oxygen in the feed according to the exemplary data and catalyst used in the '755 patent.
In two foreign references, Ermakov et al., "Transfer Processes During Polymerization of Ethylene on a Chromium Oxide Catalyst, II, The Role of Impurities in Transfer Reactions," Kinetika i Kataliz (USSR), Vol. 10, No. 2 (1969) and Dahlig et al., Khimiya i Teknologiya Polimerov, No. 4, 23 (1961), the use of oxygen (and also carbon monoxide, acetylene, carbon dioxide, and hydrogen in the Ermakov reference) is disclosed as having the effect of lowering the molecular weight of polyehtylene produced in a chromium based catalyzed polymerization of ethylene.